Cusp properties of high harmonic loops

Abstract

In determining the gravitational signal of cusps from a network of cosmic string loops, a number of key parameters have to be assumed. These include the typical number of cusps per period of string oscillation and the typical values of the sharpness parameters of left- and right-moving waves on the string, evaluated at the cusp event. Both of these are important, as the power stored in the gravitational waves emitted from the loops of string is proportional to the number of cusps per period and inversely proportional to the product of the sharpness parameters associated with the left- and right-moving modes on the string. In suitable units, both of these quantities are usually thought to be of order unity. To try and place these parameters on a more robust footing, we analyze in detail a large number of randomly chosen loops of string that can have high harmonics associated with them, such as one might expect to form by chopping off an infinite string in the early Universe. This allows us to analyze tens of thousands of loops and obtain detailed statistics on these crucial parameters. While we find in general the sharpness parameters are indeed close to unity, as assumed in previous work [with occasional exceptions where they can become $O({10}^{\ensuremath{-}2})$], the cusp number per period scales directly with the number of harmonics on the loop and can be significantly larger than unity. This opens up the possibility of larger signals than would have otherwise been expected, potentially leading to tighter bounds on the dimensionless cosmic string tension $G\ensuremath{\mu}$.